It is known that by the very principle on which they operate, helicopters are subject to high levels of vibration.
The various kinds of vibrations present, particularly when they are felt in the cabin, present numerous drawbacks, in particular in terms of crew and passenger comfort, and also in terms of fatigue of parts and equipment disposed in the cabin.
It is known that one of the main causes of such vibration is the force generated on the main lift and advance rotor by the rotation of its blades.
In particular, it is known that the fuselage of a helicopter is subjected to forces and moments (generated by the main rotor) having a reference driving frequency equal to the product of the number of blades of said rotor multiplied by its frequency of rotation. The response of the fuselage is very sensitive to the differences between the resonant frequencies of the helicopter and said driving frequency.
The vibrations as generated in this way are mainly due to the aerodynamic flow from the blades exciting resonant modes in the structure of the helicopter, and in particular the first mode in lateral bending of the helicopter tail. The vibration due to excitation of this first mode in lateral bending generally presents a frequency of a few hertz and it is extremely troublesome.
It is thus known to provide a passive vibration absorber which is generally embodied in the form of a resonator. Such an absorber is located at locations where it is desired to reduce vibrations, and it acts by being resonant at a predetermined frequency so as to reduce the amount of vibration presenting said frequency.
The vibration absorber is in the form of a damper block secured to a support such as a spring blade. One end of the support is engaged in an orifice formed in a suitable location on the structure of the helicopter, e.g. in the cabin.
Such an absorber can be adjusted by acting on the mass of the damper block, e.g. by means of washers which are added or removed by means of one or more screws secured to the block. It is also possible to adjust it by varying the distance between the block and its mount, for example by causing the block to slide along a groove provided in the support.
The absorber can be adjusted by positioning the helicopter on a vibrating bench driven at the driving frequency. A first sensor measures vibration, in other words acceleration, at the mount, and a second sensor measures acceleration at the damper block.
The phase difference between these two accelerations is measured, and when the absorber is tuned, said phase difference is equal to 90°. Otherwise, successive approximations are made by modifying the adjustment(s) of the absorber so as to come as close as possible to said desired value of 90°.
It is therefore necessary to proceed with a large number of tests on the vibrating bench before obtaining satisfactory adjustment, with the number of tests increasing with increasing precision required for the phase difference.
Documents FR 2 739 670 and U.S. Pat. No. 5,814,963 teach an improvement to the above-mentioned vibration absorber. The damper block is constituted by a main mass fixed on a spring blade having a fixed mount and an auxiliary mass whose position relative to the support is determined by a mechanical actuator such as a stepper motor. The actuator is controlled by a computer which servo-controls the position of the auxiliary mass to the phase difference between the two accelerations.
Such servo-control is relatively expensive since it requires special components. It can also present instabilities when the slope of the curve representing variation in phase difference as a function of frequency is very steep. These instabilities can lead to amplifying vibration at the driving frequency instead of reducing it.